U.S. patent application number 12/209402 was filed with the patent office on 2010-03-18 for test plug with overvoltage protection.
Invention is credited to Roy Ball, Russ Gonnam, Cristine Luong.
Application Number | 20100067157 12/209402 |
Document ID | / |
Family ID | 42007019 |
Filed Date | 2010-03-18 |
United States Patent
Application |
20100067157 |
Kind Code |
A1 |
Ball; Roy ; et al. |
March 18, 2010 |
Test Plug With Overvoltage Protection
Abstract
A test plug for use with protective relays has a circuit that
when it detects an overvoltage on the secondary side of a current
transformer limits the overvoltage amplitude and the occurrence of
the overvoltage to a predetermined number of peaks that are less
than all of the peaks that occur when the secondary side is open
circuited and the test plug is connected to the relays. The test
plug also ensures that there is a continuous flow of current in the
relays. The test plug further that has an indicator visible
external to the plug to indicate the occurrence of an open circuit
transformer secondary and that indicator remains illuminated when
the test plug is disconnected from the protective relays.
Inventors: |
Ball; Roy; (Coral Springs,
FL) ; Gonnam; Russ; (Coral Springs, FL) ;
Luong; Cristine; (Coral Springs, FL) |
Correspondence
Address: |
ABB INC.;LEGAL DEPARTMENT-4U6
29801 EUCLID AVENUE
WICKLIFFE
OH
44092
US
|
Family ID: |
42007019 |
Appl. No.: |
12/209402 |
Filed: |
September 12, 2008 |
Current U.S.
Class: |
361/91.1 |
Current CPC
Class: |
G01R 31/3277
20130101 |
Class at
Publication: |
361/91.1 |
International
Class: |
H02H 3/20 20060101
H02H003/20 |
Claims
1. A protective circuit for an electrical power system comprising:
a transformer having primary and secondary sides for converting
current at one amplitude flowing in said primary side to current of
a lower amplitude flowing in said secondary side; a protective
device; a test switch connected between said transformer secondary
side and said protective device; and a plug for testing said
circuit, said test plug inserted in said test switch so that said
secondary side current flows through said test plug, said test plug
comprising a circuit for closing said current transformer secondary
side when said secondary side is open circuited, said closing
circuit when closed maintaining a continuous flow of current
through said protective device.
2. The protective circuit of claim 1 wherein said closing circuit
comprises means responsive to said voltage across said transformer
secondary circuit exceeding a predetermined voltage level
indicative of an open secondary side for closing said open
secondary side and maintaining a continuous flow of current through
said protective device.
3. The protective circuit of claim 2 wherein said closing circuit
further comprises means for determining when said voltage across
said transformer secondary circuit exceeds a predetermined voltage
level indicative of an open secondary side.
4. The protective circuit of claim 1 wherein said closing circuit
comprises an indicator visible external to said test plug for
indicating that said test plug is connected to an open circuit
secondary side of said current transformer.
5. The protective circuit of claim 4 wherein said indicator is
energized when said test plug is connected to an open circuit
secondary side of said current transformer and said closing circuit
further comprises means for keeping said indicator energized after
said test plug is disconnected from said protective circuit.
6. A protective circuit for an electrical power system comprising:
a transformer having primary and secondary sides for converting
current at one amplitude flowing in said primary side to current of
a lower amplitude flowing in said secondary side; a protective
device; a test switch connected between said transformer secondary
side and said protective device; and a plug for testing said
circuit, said test plug inserted in said test switch so that said
secondary side current flows through said test plug, said test plug
comprising a circuit for closing said current transformer secondary
side when said secondary side is open circuited, said closing
circuit when closed limiting any overvoltage across said secondary
side to less than all of the peaks of said current that flows when
said secondary side is open circuited and said test plug is
inserted in said test switch.
7. The protective circuit of claim 6 wherein said closing circuit
comprises means responsive to said voltage across said transformer
secondary circuit exceeding a predetermined voltage level
indicative of an open secondary side for closing said open
secondary side and limiting any overvoltage across said secondary
side to less than all of the peaks of said current that flows when
said secondary side is open circuited and said test plug is
inserted in said test switch.
8. The protective circuit of claim 7 wherein said closing circuit
further comprises means for determining when said voltage across
said transformer secondary circuit exceeds a predetermined voltage
level indicative of an open secondary side.
9. The protective circuit of claim 6 wherein said closing circuit
comprises an indicator visible external to said test plug for
indicating that said test plug is connected to an open circuit
secondary side of said current transformer.
10. The protective circuit of claim 9 wherein said indicator is
energized when said test plug is connected to an open circuit
secondary side of said current transformer and said closing circuit
further comprises means for keeping said indicator energized after
said test plug is disconnected from said protective circuit.
11. A plug for testing a protective circuit for an electrical power
system, said protective circuit having a transformer with primary
and secondary sides for converting current at one amplitude at said
primary side to current of a lower amplitude at said secondary
side, a protective device and a test switch connected between said
transformer secondary side and said protective device, said test
plug comprising: a circuit for closing said current transformer
secondary side when said secondary side is open circuited, said
closing circuit when closed maintaining a continuous flow of
current through said protective device.
12. The test plug of claim 11 wherein said closing circuit
comprises means responsive to said voltage across said transformer
secondary circuit exceeding a predetermined voltage level
indicative of an open secondary side for closing said open
secondary side and maintaining a continuous flow of current through
said protective device.
13. The test plug of claim 12 wherein said closing circuit further
comprises means for determining when said voltage across said
transformer secondary circuit exceeds a predetermined voltage level
indicative of an open secondary side.
14. The test plug of claim 11 wherein said closing circuit
comprises an indicator visible external to said test plug for
indicating that said test plug is connected to an open circuit
secondary side of said current transformer.
15. The test plug of claim 14 wherein said indicator is energized
when said test plug is connected to an open circuit secondary side
of said current transformer and said closing circuit further
comprises means for keeping said indicator energized after said
test plug is disconnected from said protective circuit.
16. A plug for testing a protective circuit for an electrical power
system, said protective circuit having a transformer with primary
and secondary sides for converting current at one amplitude at said
primary side to current of a lower amplitude at said secondary
side, a protective device and a test switch connected between said
transformer secondary side and said protective device, said test
plug comprising: a circuit for closing said current transformer
secondary side when said secondary side is open circuited, said
closing circuit when closed limiting any overvoltage across said
secondary side to less than all of the peaks of said current that
flows when said secondary side is open circuited and said test plug
is inserted in said test switch.
17. The test plug of claim 16 wherein said closing circuit
comprises means responsive to said voltage across said transformer
secondary circuit exceeding a predetermined voltage level
indicative of an open secondary side for closing said open
secondary side and limiting any overvoltage across said secondary
side to less than all of the peaks of said current that flows when
said secondary side is open circuited and said test plug is
inserted in said test switch.
18. The test plug of claim 17 wherein said closing circuit further
comprises means for determining when said voltage across said
transformer secondary circuit exceeds a predetermined voltage level
indicative of an open secondary side.
19. The test plug of claim 16 wherein said closing circuit
comprises an indicator visible external to said test plug for
indicating that said test plug is connected to an open circuit
secondary side of said current transformer.
20. The test plug of claim 19 wherein said indicator is energized
when said test plug is connected to an open circuit secondary side
of said current transformer and said closing circuit further
comprises means for keeping said indicator energized after said
test plug is disconnected from said protective circuit.
Description
FIELD OF THE INVENTION
[0001] This invention relates to test plugs used with protective
relays and more particularly to providing protection against the
overvoltages and other conditions that occur during an unplanned
opening of a current transformer.
DESCRIPTION OF THE PRIOR ART
[0002] Protective relays are devices designed to protect the
electrical power system during disturbances. To accomplish their
functions they take signals from the power system mainly by way of
two types of devices, the potential transformer and current
transformers.
[0003] Potential transformers convert high voltage levels down to
low levels. Current transformers convert high current levels to low
current levels. The lower level signals are then read by devices
like protective relays and demand meters.
[0004] It is well known in the industry that current transformers
have a unique operating characteristic. If the current transformer
is operating under load (current flowing in the primary) and the
low level secondary side circuit is opened, high voltages develop
at the current transformer secondary terminals which can be
dangerous or fatal to operators and may damage devices connected to
the current transformer circuits.
[0005] The opening of a current transformer secondary side circuit
could happen because of an accidental disconnection, incorrect test
setup, defective equipment and leads, or when measuring the current
the demand meter has fuse protection and the fuse operates to leave
the circuit open.
[0006] To facilitate maintenance and testing, test switches are
usually installed between the current transformers and the
protective and control devices which may for example be relays.
FIG. 1 shows the typical circuit 10 that connects the current
transformer 12, and the protection and control devices 14 which in
this instance is a relay. The test switch 16 is between the
transformer 12 and the relay 14. The arrows show the flow of
current in the circuit 10. The test switches 16 along with the use
of test plugs as shown in FIG. 2, described below, facilitate
access to the electrical connections of both the protective relays
14 and demand meters (not shown in FIG. 1).
[0007] FIG. 2 shows a test plug 18 inserted in the test switch 16
of FIG. 1 and a meter 20 connected to the test plug 18. The purpose
of the test plug 18 is to facilitate measurement, calibration and
troubleshooting. Inserting the test plug with the relay staying in
service diverts the current to pass through the test plug allowing
the meter to take the measurement while keeping the relay in
service.
[0008] FIG. 3 shows an example of an unplanned opening of the
current transformer circuit 10. In this example, the leads from the
test plug 18 to the meter 20 are disconnected, but other conditions
could cause the current transformer circuit to open. There is no
current flow and a high voltage is developed at the secondary of
the current transformer 12 due to the opening of the current
transformer circuit. This high voltage can be several thousands of
volts and has been measured in some instances as high as 15 kV.
With this high voltage there is a safety concern for the user of
the test switch 18 and meter 20 and a reliability concern for the
equipment connected to the transformer which in this example is a
relay. Also, the lack of current flow causes an incorrect
interpretation by the protection and control equipment 14 of the
true conditions on the power system. As a result of the incorrect
information the protection and control equipment 14 could operate
incorrectly or fail to operate when it is needed to operate.
[0009] Further the current transformer 12 may magnetize when its
secondary side is open circuited. If the current transformer
magnetizes it may still be magnetized when the open circuit is
removed from the current transformer circuit. Thus the current
flowing through the secondary may have an incorrect amplitude and
an incorrect phase angle for that current.
[0010] The prior art provides overvoltage protection against the
unplanned opening of current transformers in the test plug and
meter section of the circuit. This is accomplished by continuous
monitoring of the voltage levels. If an overvoltage condition is
developed an electronic component is always placed in parallel with
the main current path. In response to the overvoltage condition,
the electronic component switches on bypassing the open circuit,
restoring the current flow and eliminating the overvoltage
conditions. Thus the prior art limits the voltage to safer levels
of under 100 volts.
[0011] As the voltage goes down from the voltage present upon the
occurrence of an overvoltage condition to safer levels and the
current flow is restored via the electronic switching device, there
is less danger for the users of the test plug. The new NFPA 70E
standards state that any voltage above 48 volts should be carefully
analyzed and where required the user should wear personal
protective equipment. OSHA says that 50 Vac is a hazard [see 29 CFR
1910.333(a) (1) and 1910.269(I) (1)] and the IEC (International
Electrotechnical Commission)says that a hazard is 30 Vac RMS, 42
VAc peak, or 60 VDC.
[0012] When the alternating current flow crosses a zero value the
prior art electronic switching device turns off eliminating the
bypass of the open circuit. The current continues to flow in the
primary (high side) of the current transformer. With the secondary
of the transformer open circuited an overvoltage condition again
develops across the secondary and the protection on the test plug
once again limits the voltage to safer levels and the cycle
continues to repeat until the test plug is removed from the
circuit. Thus until the test plug is removed from the circuit, the
user of the test plug is continuously exposed to voltages that may
be dangerous.
[0013] While the prior art solution keeps the secondary voltages to
safer levels, it does so by continuous on and off cycles. These
cycles cause a lack of continuity of current to the protective
relays and also negatively impacts the integrity of the current
waveform as that waveform is distorted in the secondary of the
current transformers. That lack of either continuity and integrity
in the secondary current could be seen by some protective relays as
a disturbance in the electrical power system, causing the
protective relay to take the power system out of service.
SUMMARY OF THE INVENTION
[0014] A protective circuit for an electrical power system has:
[0015] a transformer having primary and secondary sides for
converting current at one amplitude flowing in the primary side to
current of a lower amplitude flowing in the secondary side;
[0016] a protective device;
[0017] a test switch connected between the transformer secondary
side and the protective device; and
[0018] a plug for testing the circuit, the test plug inserted in
the test switch so that the secondary side current flows through
the test plug, the test plug has a circuit for closing the current
transformer secondary side when the secondary side is open
circuited, the closing circuit when closed maintaining a continuous
flow of current through the protective device.
[0019] A protective circuit for an electrical power system has:
[0020] a transformer having primary and secondary sides for
converting current at one amplitude flowing in the primary side to
current of a lower amplitude flowing in the secondary side;
[0021] a protective device;
[0022] a test switch connected between the transformer secondary
side and the protective device; and
[0023] a plug for testing the circuit, the test plug inserted in
the test switch so that the secondary side current flows through
the test plug, the test plug has a circuit for closing the current
transformer secondary side when the secondary side is open
circuited, the closing circuit when closed limiting any overvoltage
across the secondary side to less than all of the peaks of the
current that flows when the secondary side is open circuited and
the test plug is inserted in the test switch.
[0024] A plug for testing a protective circuit for an electrical
power system, the protective circuit having a transformer with
primary and secondary sides for converting current at one amplitude
at the primary side to current of a lower amplitude at the
secondary side, a protective device and a test switch connected
between the transformer secondary side and the protective device,
the test plug has:
[0025] a circuit for closing the current transformer secondary side
when the secondary side is open circuited, the closing circuit when
closed maintaining a continuous flow of current through the
protective device.
[0026] A plug for testing a protective circuit for an electrical
power system, the protective circuit having a transformer with
primary and secondary sides for converting current at one amplitude
at the primary side to current of a lower amplitude at the
secondary side, a protective device and a test switch connected
between the transformer secondary side and the protective device,
the test plug has:
[0027] a circuit for closing the current transformer secondary side
when the secondary side is open circuited, the closing circuit when
closed limiting any overvoltage across the secondary side to less
than all of the peaks of the current that flows when the secondary
side is open circuited and the test plug is inserted in the test
switch.
DESCRIPTION OF THE DRAWING
[0028] FIG. 1 shows the typical connection between the current
transformer and the protection and control devices.
[0029] FIG. 2 shows a test plug inserted in the test switch of FIG.
1 and a meter connected to a prior art test plug.
[0030] FIG. 3 shows an example of an unplanned opening of the
current transformer circuit.
[0031] FIGS. 4a and 4b show an example of the voltage and current
waveforms for the prior art test plug.
[0032] FIGS. 4c and 4d show the voltage and current waveforms for
the test plug of the present invention.
[0033] FIG. 5 shows the typical connection of the test plug of the
present invention to the current transformer circuit.
[0034] FIG. 6 shows a circuit schematic for one embodiment of the
electronic circuit that is in the test plug of the present
invention.
DETAILED DESCRIPTION
[0035] Referring now to FIGS. 4a to 4d, there are shown in FIGS. 4a
and 4b an example of the voltage and current 30 waveforms for the
prior art solution and in FIGS. 4c and 4d the voltage and current
waveforms for the present invention. FIG. 4a shows that the prior
art solution limits the overvoltage peak amplitude in each half
cycle to a maximum of 100 volts. Thus the user of a prior art test
plug is exposed each half cycle of the AC voltage (every 8.33
millisecond for a voltage whose frequency is 60 Hertz) to an
overvoltage peak amplitude having a 100 volt maximum.
[0036] In contrast thereto and as is shown in the overvoltage
waveform in FIG. 4c, the test plug of the present invention
substantially limits the amplitude of the overvoltage as compared
to the prior art solution. Tests have shown that the test plug of
the present invention limits the overvoltage peak amplitude to a
maximum of 35 volts at the moment of the opening of the CT. As is
shown in FIG. 4c, the overvoltage limiting by the test plug of the
present invention is for one pulse only as thereafter the test plug
of the present invention fully eliminates any overvoltage
condition.
[0037] As can be seen by comparing the current waveform of the
prior art solution shown on FIG. 4b with the current waveform in
FIG. 4d for the test plug of the present invention, the present
invention maintains the continuity and integrity of the current as
compared to the prior art solution. Tests have shown that the test
plug of the present invention may cause a minimal lack of
continuity and integrity in the current waveform. These tests have
also shown that if the lack of continuity and integrity does occur
it is as is shown in FIG. 4d limited to the first half cycle while
as can be seen in FIG. 4c providing continuous safe overvoltage
protection for the end user of the test plug.
[0038] Thus the test plug of the present invention eliminates the
possibility of relay misoperations at the same time that it
provides safety to the user by eliminating the excessive voltage
levels. This test plug eliminates or reduces the duration of the
distortion in the current waveform to the first half cycle while
providing continuous safe over-voltage protection for the end user
and eliminates the lost signal condition.
[0039] Referring now to FIG. 6, there is shown a circuit schematic
for one embodiment of the electronic circuit that is in the test
plug of the present invention. The test plug 22 of the present
invention is typically connected as shown in FIG. 5 to the current
transformer circuit in series. A current meter 20, also shown in
FIG. 5, is used to monitor current levels and is connected to the
connection points identified in FIG. 6 as banana plug 1 BP1 and
banana plug 2 BP2. While FIG. 5 is identical to FIG. 3 described
above, it should be appreciated that the test plug 22 shown in FIG.
5 is the test plug of the present invention.
[0040] Referring once again to FIG. 6, upon the occurrence of an
unplanned opening of the current transformer (CT) secondary side,
an overvoltage condition is developed across the test plug
terminals BP1 and BP2. When the voltage level across terminals BP1
and BP2 exceeds the threshold value of diac D3, the current flows
through resistor R1 and phototriac U1 or U2, depending on the
instantaneous polarity of the voltage waveform. The phototriac U1
or U2 in turn closes contacts U1 or U2 terminals 3 and 4, allowing
the circuit that includes battery BT to be completed and turn on
the combination of triacs Q1 and Q2.
[0041] The turning on of Q1 and Q2 short circuits the current flow
and thus "restores" the open CT circuit.
[0042] The battery BT also powers up a miniature power relay K1.
The miniature power relay K1 closes its contacts and carries most
of the current. This arrangement is used to eliminate the need for
the extensive heat sink required for Q2 that would be needed for Q2
in the absence of relay K1. The arrangement also allows for a
smaller package, less components and lower cost.
[0043] The battery BT also provides current to power up the LED
indicator D1 after the relay K1 closes its contacts.
[0044] The LED indication remains on even after the test plug is
removed from the open circuit. This shows that the circuit to which
the plug was connected had an open CT secondary. If the test setup
is unattended for some reason, the LED indication alerts the user
of the condition upon his/her return.
[0045] Set and reset buttons collectively identified in FIG. 6 as
S1 are provided to test the test plug 22 prior to each use. The
testing shows that the circuit in the test plug 22 will function
correctly if the CT secondary opens.
[0046] It should be appreciated that while FIG. 6 shows one
embodiment for the circuit in test plug 22 that other embodiments
for that circuit will accomplish the same result.
[0047] It is to be understood that the description of the foregoing
exemplary embodiment(s) is (are) intended to be only illustrative,
rather than exhaustive, of the present invention. Those of ordinary
skill will be able to make certain additions, deletions, and/or
modifications to the embodiment(s) of the disclosed subject matter
without departing from the spirit of the invention or its scope, as
defined by the appended claims.
* * * * *